Update the Swift badge

Author: groue

README.md

@@ -10,7 +10,7 @@
 </p>
 
 <p align="center">
-    <a href="https://developer.apple.com/swift/"><img alt="Swift 5.7" src="https://img.shields.io/badge/swift-5.7-orange.svg?style=flat"></a>
+    <a href="https://developer.apple.com/swift/"><img alt="Swift 6" src="https://img.shields.io/badge/swift-6-orange.svg?style=flat"></a>
     <a href="https://github.com/groue/GRDB.swift/blob/master/LICENSE"><img alt="License" src="https://img.shields.io/github/license/groue/GRDB.swift.svg?maxAge=2592000"></a>
     <a href="https://github.com/groue/GRDB.swift/actions/workflows/CI.yml"><img alt="CI Status" src="https://github.com/groue/GRDB.swift/actions/workflows/CI.yml/badge.svg?branch=master"></a>
 </p>